docgen: for some reason array square brackets are always trimmed

As found during the review of #150598, it appears that we unconditionally trim the array square brackets from the type name, so even if a builtin function takes an array as the argument, the array element type will be shown in the markdown. I think it's because of this line https://github.com/cockroachdb/cockroach/blob/064aa707cdc0283bea483b513e6d6df97219ca93/pkg/cmd/docgen/funcs.go#L276

For example, as of that PR we'll support `ltree[] ?<@ ltree`, yet `ltree ?<@ ltree` is shown in the markdown.

This was added in ancient times in #19153, and I don't see any discussion around it. We should figure out why that trimming logic is there and adjust it somehow so that proper arrays are shown when applicable.

Jira issue: CRDB-53714